I may have inadvertently discovered the answer to the great airbrush mystery. On the 72nd Scale Aircraft board there was a discussion concerning which airbrush everyone was using and why. The discussion wandered, as these things are prone to do, and eventually touched on thinners. Keith Rider, one of the board members, mentioned that his nozzle o-ring was destroyed by lacquer thinner. And I use lacquer thinner almost exclusively for airbrush work.
And that set the wheels to turning. Though I usually break down the nozzle for a cleaning after each session, I honestly could not remember seeing an o-ring in there. So I took the thing apart, and sure enough, no o-ring. Going to the Iwata website, I confirmed that the Eclipse HP-CS should indeed have said o-ring as part of the nozzle assembly.
So is this the origin point of my airbrush woes? Did the ring dissolve or fall out during one of the many cleaning sessions? No clue as of yet, but I do have a source for a replacement part, and will have one flying toward me within hours. I really should pick up a new nozzle tip and needle at the same time, but will try to get the new o-ring in place first.
So stay tuned; I don't think this story is over yet, but at least we may have some progress!